The present invention relates to a method of controlling a DC/DC converter, which converts a DC input from a DC power supply to an arbitrary DC output through a transformer. Specifically, the present invention relates to a method of controlling a DC/DC converter in the state of light load including the state of no load.
FIG. 9 is a circuit diagram of a conventional DC/DC converter. FIG. 10 is a wave chart for explaining the operation of the conventional DC/DC converter of FIG. 9. The conventional DC/DC converter in FIG. 9 is a flyback converter, which modifies the width of the ON-pulses of a switching element 2. In other words, the flyback converter conducts pulse width modulation (PWM) to make the output voltage Vo thereof coincide with a reference voltage VREF. A detecting and adjusting circuit 6 for detecting and adjusting the output voltage of the converter outputs a reference voltage Vc. A first triangular-wave generating circuit 8 outputs an output voltage Vr. A first comparator 7 compares the output voltage Vr with the reference voltage VREF, and outputs a PWM signal VM. The PWM control switches on the switching element 2 in case of Vc greater than Vr, and switches off the switching element 2 in case of Vc less than Vr.
A rectangular-wave generating circuit (hereinafter referred to as an xe2x80x9coscillator circuitxe2x80x9d) 23 outputs an output Vs at a fixed frequency and at a fixed duty ratio. A gate drive circuit 9 drives the switching element 2 based on a drive signal VG, which is the logical integration of the PWM signal VM and the output Vs from the oscillator circuit 23. Therefore, the switching element 2 is driven intermittently such that the switching element 2 repeats switching during the oscillating period when the output Vs from the oscillator circuit 23 is at the high level (H-level), and the switching element 2 is forced to stop switching during the forced stop period when the output Vs from the oscillator circuit 23 is at the low level (L-level).
The forced stop period facilitates decreasing the number of switching per a unit period of time to reduce the switching loss and the conduction loss. However, since no energy is supplied to the output of the flyback converter during the forced stop period, the output voltage from the flyback converter causes pulsation slightly. Therefore, it is necessary to set the oscillating period and the forced stop period appropriately so that the pulsation of the output voltage caused by the maximum load predicted in the state of light load (hereinafter referred to as the xe2x80x9cthe maximum light loadxe2x80x9d) may be confined within an allowable range.
Since the oscillating period and the forced stop period are controlled at a fixed frequency and a fixed duty ratio in the conventional DC/DC converter shown in FIG. 9, it is necessary to set the oscillating period and the forced stop period appropriately so that the pulsation of the output voltage caused by the maximum light load may be confined within an allowable range. As a result, useless switching is caused more often as the load becomes lighter. As the number of the useless switching increases, the switching loss and conduction loss increase. When the ON-pulse width (ON-period) of the switching element changes abruptly at the changeover timing of the oscillating period and the forced stop period, electrical stress is exerted abruptly to the transformer. When the oscillating frequency of the oscillator circuit 23 is within the audible range, very noisy sounds are caused from the transformer.
In view of the foregoing, it is an object of the invention to provide a DC/DC converter, which facilitates reducing the electric power consumption in the state of light load and preventing very noisy sounds from the transformer.
Further objects and advantages of the invention will be apparent from the following description of the invention.
According to the invention, there is provided a method of controlling a DC/DC converter, wherein a switching element is switched on and off to convert a voltage supplied from a DC power supply to a constant DC voltage, the method setting an oscillating period and a forced stop period of the switching element in the state of light load including the state of no load to make the switching element work intermittently. The method includes: using an output signal from an output voltage detecting and adjusting circuit, which controls the output of the DC/DC converter at the constant DC voltage, as a first reference signal for determining the on-off timing of the switching element; and using a comparison signal obtained by comparing the first reference signal and a carrier signal of a constant frequency as a control signal for controlling the oscillating period and the forced stop period of the switching element in the state of light load including the state of no load. Advantageously, the carrier signal is a triangular wave or a saw-tooth wave.
Advantageously, the method of controlling the DC/DC converter further includes inputting the first reference signal to an impedance conversion element; using an output from the impedance conversion element as a second reference signal for determining the on-off timing of the switching element; and making the comparison signal lower the second reference signal to control the forced stop period of the switching element. Advantageously, the impedance conversion element is a voltage follower.
Advantageously, the method of controlling the DC/DC converter further includes charging a capacitor at a constant current at a first changeover timing when the forced stop period is changed over to the oscillating period, by using the control signal for controlling the oscillating period and the forced stop period of the switching element; discharging the capacitor at a constant current at a second changeover timing when the oscillating period is changed over to the forced stop period, by using the control signal for controlling the oscillating period and the forced stop period of the switching element; comparing the voltage of the capacitor and the first reference signal with each other; and using smaller one of the voltage of the capacitor and the first reference signal as a new control signal for determining the on-off timing of the switching element.